Biological Journal ofthe L.innean Socicly (1992), 45: 255-269. With 1 figure
An ecological classification of odonate mating
systems: the relative influence of natural, interand intra-sexual selection on males
KELVIN F. CONRAD* AND GORDON PRITCHARD
Division of Ecology, Department of Biological Sciences, The University of Calgary,
Calgary, AB, Canada T2.N IN4
Received 22 August 1990, accepted for publication 2 Noumber 1990
We separate the mating systems of odonates into two main groups: non-resource and resource-based
systems. These two groups comprise five classes of mating system: encounter-limited mating, free
female choice, resource-limitation, resource-control and female-control. These classes are consistent
with previous classifications of odonate mating systems and with the overall classification of mating
systems by Emlen & Oring (1977:Science, 197:215-223). Whereas Emlen & Oring’s classification of
mating systems was concerned with differences in sexual selection between mating systems, our
classification of odonate mating systems also addresses the influence of inter- and intra-sexual
selection on males within a mating system. Predictions about such relationships are useful in
multivariate analysis of odonate lifetime reproduction success. Among most odonate mating systems,
much of the sexual selection on males results from male-male competition for access to mates.
Sexual selection via female choice is relatively less important or operates indirectly through females’
choices of times or places to mate. We place resource-control and resource-limitation at opposite
ends of a resource-defence continuum and postulate female choice will have greater influence in
mating systems that are more like a resource-limitation system and less influence in mating systems
that are more like resource-control. Sexual selection is likely to be weak in species that resort to
encounter-limited mating where longevity is likely to contribute strongly to variation in
reproductive success. Females have limited opportunity to exercise choice among males in the
female-control mating system and in this system selection is most likely to operate on male characters
which contribute to their efficiency in searching for and capturing mates. Predictions about the
differences in the intensity of sexual selection between different odonate mating systems should be
made on the basis of the variation in the number of potential fertilizations per male or even per
ejaculate, rather than the number of fertilizable females per male. Very different mating systems
could result in similar patterns of variation in male reproductive success.
KEY WORDS:-Odonate
female choice.
mating systems - sexual selection - resource-control - female-control -
Introduction . . . . . . .
Odonates are promiscuous.
. .
Previous classification schemes.
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The influence of sperm competition
A model of odonate mating systems
.
Non-resource based systems . .
Resource-based systems . . .
Comparisons between mating systems .
Conclusions . . . . . . .
Acknowledgements
. . . . .
References
. . . . . . .
CONTENTS
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Present Address: Department of Biology, Acadia’s University, Wolfville, NS, Canada BOP
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256
K. F. CONRAD AND G . PRITCHARD
INTRODUCTION
Emlen & Oring (1977) provided an ecological classification of animal mating
systems that has been applied to a wide variety of organisms. The scheme and its
accompanying terminology are both general and descriptive enough to serve
sufficiently in most discussions of mating systems. However, as Emlen & Oring
(1977: 215) point out, their principal objective was to address avian mating
systems and they recognize that their scheme might not apply so well to insects.
Recently, odonatologists have begun to study the ultimate causes of the social
interactions that constitute odonate mating systems. They have done this by
attempting to partition the variation in lifetime reproductive success into that
which is influenced by natural selection, by intrasexual selection (male-male
competition) and by intersexual selection (female choice of mates) (e.g. Koenig
& Albano, 1987; Fincke, 1986, 1988; Hafernik & Garrison, 1986; McVey, 1988).
These studies have been performed using the powerful regression techniques
developed by Wade & Arnold (1980) and Arnold & Wade (1984). These
techniques, however, are still largely descriptive and can be misleading unless
they are applied with specific hypotheses in mind (Grafen, 1988; Koenig &
Albano, 1986). Whereas Emlen & Oring (1977) dealt with the general
relationships of temporal and spatial distribution of resources, the availability of
mates and the pattern of sexual selection compared between mating systems, the
new studies of lifetime reproductive success require specific predictions about the
relative intensity of intra- and inter-sexual selection and the kinds of characters
on which sexual selection operates within each type of mating system.
Our intention in this paper is to use the framework of Emlen & Oring (1977)
to develop predictions about the relative influence of intra- and inter-sexual
selection on the evolution of different odonate mating systems.
Odonates are promiscuous
Emlen & Oring (1977) viewed mating systems mainly from the perspective of
monogamy versus polygamy. They discussed the “environmental potential for
polygamy” and noted that polygamy is more prevalent in species where one sex
is freed from parental care duties (p. 216). However, when both sexes are freed
from parental care, as in dragonflies, both sexes have a high potential for
polygamy and the mating system is best described as ‘promiscuous’. Mating in
dragonflies consists of an encounter, sometimes brief courtship, copulation and
oviposition, which is often accompanied by mate-guarding. The association
between members of a mated pair may range from a few minutes to a few hours
after which either member of the pair can mate again with a different individual.
Parental investment by either sex largely consists of energy invested in gametes.
Hence, odonates are promiscuous. Even females of the damselfly lschnura verticalis
Say, that are “effectively monogamous” (Fincke, 1987), sometimes do mate
more than once, as do the males of the species. The mating systems of odonates
cannot be viewed from the dichotomy of monogamy versus polygamy.
Promiscuity is the dominant pattern of mating among odonates.
Previous classijication schemes
Previously, workers have attempted to classify odonate mating systems based
on several different ecological criteria (Table 1). Most of the schemes deal with
CLASSIFICATION OF O W N A T E MATING SYSTEMS
257
TABLE
1. Criteria used in previous classification schemes
Criteria used
Author(s)
Duration of copulation
Method of encounter of sexes
Degree of territoriality
Spatial dispenion of oviposition sites and females
resulting from larval requirements
Pattern of temporal availability of females
Oviposition behaviour of females
Corbet (1962)
Waage ( 1984a)
Campanella (1975)
Buskirk & Sherman (1985)
Poethke & Kaiser (1985, 1987)
Rowe (1988)
the availability (spatial and temporal distribution) of two resources: oviposition
sites and mates. It is not surprising that these classification schemes all separate
species similarly, since the criteria used in each scheme are correlated. All of the
previous schemes are descriptive, but do not allow the prediction of the type of
mating system of a species, based on the knowledge of a few ecological characters
of the species.
Taking an overview of the different classifications of odonate reproductive
behaviour by Corbet ( 1962), Waage ( 1984a), Campanella ( 1975), Buskirk &
Sherman ( 1985) and Poethke & Kaiser ( 1985, 1987) (see Table 1 ) ) the following
patterns emerge:
(1) In species having oviposition sites with a clumped distribution, males are
territorial (Buskirk & Sherman, 1985; Campanella, 1975; Sherman, 1983;
Waage, 1984a).
(2) Territorial species copulate within or very near the territory and those that
are not territorial frequently copulate away from the water, or if they encounter
mates at the water, they move a short distance away to copulate (Corbet, 1962;
Waage, 1984a).
(3) Species that copulate at or near the oviposition site have brief copulations
and species that copulate away from the oviposition site have long copulatory
durations (Corbet, 1962; Waage, 1984a).
(4) Species having long copulatory durations display either contact guarding,
or no guarding at all, and species with short copulatory durations usually display
non-contact guarding (Waage, 1984a).
(5) Where females arrive at the breeding site predictably and oviposit quickly,
males should be territorial and where the arrival of females is unpredictable and
oviposition takes a long time, males should be non-territorial (Poethke & Kaiser,
1987).
These five points should appear axiomatic to odonatologists. They are
presented here to emphasize the relationship among oviposition site (resource)
distribution, copulatory duration, mate-guarding tactics, location of copulation
relative to the oviposition site and the type of oviposition performed. These
relationships also form the basic tenets of our classification of mating systems.
The influence of sperm competition
In the absence of parental care, it is not unreasonable to expect both sexes to
mate frequently and with different mates, unless there are some limitations of
time or physiology (e.g. the nuptial flight of a mayfly). In odonates, promiscuity
258
K. F. CONRAD AND G. PRITCHARD
is related to the widespread occurrence of sperm competition (Waage, 1979a,
1982, 1984a, 1986a,b). The sperm a female carries is replaced (or at least
displaced) each time she mates and each copulation is profitable to a male,
regardless of whether his mate had mated before. Also, there is strong selection
for male behaviour that prevents their mates from being mated again before they
have oviposited, giving rise to several types of mate guarding. In the absence of
effective sperm precedence, odonates would most likely be truly monogamous,
since any mating after the first would not be profitable to either sex (assuming
one mating could provide enough sperm to fertilize all eggs a female can lay in a
lifetime, cf. Grieve, 1937; Fincke, 1987).
A MODEL OF ODONATE MATING SYSTEMS
Non-resource-based systems
Mating systems should be viewed as a product of the interaction of inter- and
intra-sexual selection. As Emlen & Oring (1977) and Borgia (1979) have noted,
when a resource such as an oviposition site, is necessary to females for
reproduction, males should attempt to monopolize this resource to gain access to
multiple females. When male odonates are unable to influence female mating
behaviour through some form of resource defence, three types of mating systems
can occur: free female choice, female-control and encounter-limited mating.
Free f m a l e choice
In the most basic mating system where there is no parental care, males should
display their genetic quality using a behavioural or morphological trait that is
phenotypically an honest signal of genetic quality (cf. Kodric-Brown & Brown,
1984). Females should be able to survey males, choose the genetically most
superior male and mate with him Uanetos, 1980). The superior genetic material
is the male’s only contribution to the offspring and thus is his only contribution
to his and his mate’s fitness. We refer to this system as ‘free female choice’.
The free female choice mating strategy should be found in species where the
co-occurrence of males and females is unpredictable but common, and males are
unable to control female access to oviposition sites because either females are farranging and oviposition sites are widely distributed or oviposition sites cover
greater areas than males are able to patrol. Because of their low association with
the oviposition site, these species are expected to mate far from the oviposition
site and are therefore expected to spend relatively long periods in copulation
(Corbet, 1962, 1980; Waage, 1984a). Under such conditions, males are expected
to evolve elaborate morphological or behavioural signals for females to use as
criteria for selecting a mate. Variation in the quality of these displays should be
strongly correlated with variation in male reproductive success and intersexual
selection (female choice of mates) should be more intense than intrasexual
selection (male-male competition for access to mates). Such a mating system
might best be described by ‘good genes’ models of sexual selection. However,
mating systems that are exclusively free female choice probably do not exist
among odonates. Few species, if any at all, rely solely on elaborate mating
displays, exclusive of territories, to attract mates.
CLASSIFICATION OF ODONATE MATING SYSTEMS
259
Female-control
When the co-occurrence of males and females is predictable in time and space,
but males are unable to control female access to oviposition sites, males may
resort to attempting to control females themselves and thereby coerce females to
mate by practising a female-control strategy. This is similar to the femaledefence polygyny described by Emlen & Oring (1977), although males defend
only one mate at a time (see also Convey, 1989), and to the male-controlled
system described by Borgia (1979, 1981). In order to parallel the idea of
resource-control, we refer to this strategy as ‘female-control,.
In such a mating system, the success of males depends on their ability to
position themselves appropriately to encounter females that are on their way to
oviposit. We assume females signal their readiness to mate by approaching the
breeding area. The encounters between individual males and females are likely
to occur more-or-less at random and oviposition sites are numerous and widely
distributed within a limited area. Under such conditions, and especially when
the species is not very mobile, (e.g. most Zygoptera), males have little
opportunity to control female access to oviposition sites.
Males must actively seek mates, either by searching for them or by
maintaining a position to intercept females as they approach the oviposition
area. Males capture females and detain them, which prevents other males from
mating with them (see also Borgia, 1981). There is no courtship display. Female
choice is restricted to accepting or rejecting males that capture them or to the
indirect method of choosing the time of day to approach the water when the
‘best’ males are most likely to capture them. These species mate away from the
oviposition site and therefore copulatory duration is predicted to be long (cf.
Corbet, 1962). Males remain in tandem with females until they have finished
ovipositing (Bick & Bick, 1965) or submerge to oviposit and are no longer
susceptible to takeover by other males (Fincke, 1985). The time a male and
female are in association with one another is long and multiple matings within a
day should be rare (see, for example, Bick & Bick, 1965). Most zygopterans, with
the obvious exception of territorial species such as the calopterygids and
chlorocyphids, and the possible exception of some members of the genus lschnura
(Rowe, 1978; Fincke, 1987), probably practise the female-control strategy.
Conrad & Pritchard (1988) discuss Argiu uiuida Hagen as a specific example.
Sexual selection on males should have its greatest effects on male traits that
allow males to obtain the best position to encounter females quickly when they
appear. Strong competition among males for first access to females should mean
that intrasexual selection is more intense than intersexual selection in such a
mating system. Sexual selection on males should be on traits that affect their
mate-seeking ability, rather than their ability in direct physical contests between
males, because mate-seeking is not localized around a defensible resource. Such
traits might include increased thermoregulatory ability or a low minimum
threshold for flight to allow males to seek mates over a wider range of
temperatures, or increased foraging efficiency. These provide males with the
ability to seek mates for greater periods of time, or begin their search earlier in
the day.
The supposition that male odonates can entice unwilling females to mate is
central to this mating system. It has been commonly proposed that male
damselflies are unable to force females to mate because females must take an
260
K. F. CONRAD AND G . PRITCHARD
active role in the formation of the wheel position. In the absence of any as yet
undiscovered ‘hard-wired’ response to male courtship or the tandem, males
indeed cannot physically force females to copulate. However, instead of using
physical force, male odonates may be able to coerce females to mate with them.
In the female-control mating system (as in all systems), males obtain mates by
grasping them with their terminal appendages to form the tandem position.
Females may refuse to form the tandem or later, the wheel, but they cannot
easily escape the tandem once it is formed. Hilton (1984) observed female
Nehalennia gracilis Morse in tandem with headless males. Miller (1987a) and
Miller & Miller (1981) found that only male Ischnura elegans (Vander Linden)
could initiate or terminate the tandem position and Miller (1987b) found that
male I. elegans controlled the duration of copulation once the wheel was formed.
In a female-control mating system a female approaching the water is captured
by a male. She may copulate with the male and oviposit with him guarding, or,
she may resist him, escape the tandem and mate with another male or continue
to the water and attempt to oviposit unguarded. Because, typically, there are so
many other males at the oviposition site, a female that avoids all matings after
her first insemination will not be able to oviposit unharassed. Furthermore,
because it is difficult for females to escape once taken in tandem, they must
devote time and energy and risk injury to escape males that take them in
tandem. Even females that simply remain passive must still invest time in their
refusal. Females should refuse to mate only when the benefits of finding a new
mate or remaining unguarded, measured against the value of the current mate,
exceed the costs of refusing to mate. When the costs of refusing to mate exceed
the benefits of refusal, males are able to coerce females to mate (Parker, 1970, see
also Rowe, 1988). At the population level, this has the effect of reducing the
variation in the value of males to female reproductive success.
Encounter-limited mating
Finally, the potential for males to monopolize females does not necessarily lead
to the evolution of complex mating systems. If male-female encounters are rare
relative to an individual’s lifespan and the probability of mating multiply in a
lifetime is low, the best mating strategy is for members of either sex to mate with
the first individual of the opposite sex they encounter to ensure that at least some
offspring will be produced. We call the pattern of mating that results in these
conditions ‘Encounter-limited mating’.
The encounter-limited mating strategy should be found in species where males
are unable to control female access to oviposition sites because male-female
encounters are infrequent and either females are far-ranging and oviposition sites
are widely distributed or oviposition sites cover greater areas than males are able
to patrol. Such species might include large, far-ranging dragonflies, or
zygopterans that usually occur in very low densities. These species should mate
far from the oviposition site and spend relatively long periods in copulation (cf.
Corbet, 1962, 1980; Waage, 1984a).
Mating should occur more-or-less at random with respect to male and female
characteristics. The character of individuals that should influence lifetime
reproductive success most strongly is longevity, since those individuals that live
longest have the greatest chance of encountering the most mates. Elaborate
CLASSIFICATION OF ODONATE MATING SYSTEMS
261
morphological or behavioural mating signals are not expected in species that
practise encounter-limited mating.
Encounter-limited mating is presented here, not so much as distinct mating
system, but as a possibly influential component of other mating systems where
mating occurs infrequently. Through manipulations, or by observing
populations of the same species at different densities, it should be possible to
discover a threshold density below which discrimination among mates on the
basis of behavioural or morphological characters is virtually non-existent.
Resource-based systems
The ability of males to control a resource needed by females for reproduction
can lead to various forms of resource-defence polygamy and territorial behaviour
in males (Emlen & Oring, 1977). Among odonates, however, the association
between males, females and oviposition sites (the needed resource) is only brief,
The oviposition site serves as a rendezvous for males and females but a mated
pair does not need to maintain a territory around the oviposition site to ensure
the success of their offspring. Because female d o n a t e s are able to store viable
sperm for some time (Grieve, 1937; Fincke, 1987), non-virginal females are able
to use oviposition sites and successfully produce offspring without further male
intervention. This means that various types of resource-defence promiscuity
may be placed along a continuum, depending on the ability of the population of
males to prevent females from ovipositing without first copulating with a
territorial male. We will consider the two extremes of this continuum: resource
control and resource limitation.
Resource-contro I
In the resource control system (cf. Borgia, 1979), males are able to control the
access of females to all available oviposition sites, or at least all the sites females
would benefit from seeking. In such a case, a female must mate with a male that
holds a territory in order to be able to oviposit.
When oviposition sites are clumped and males are extremely mobile,
territorial males may be able to control all female access to oviposition sites.
Females should mate with the male occupying the best territory and therefore
competition among males to hold the best territory should be intense. Sexual
selection should favour male traits that allow males to win the territory and hold
it, and intrasexual selection should be more intense than intersexual selection.
Large male size, or a high flight-muscle ratio (Marden, 1989) for example, might
be favoured if it conveyed an advantage in battles for territories,
Male density and the probability of male interference with oviposition are
important to the degree of control males have over female access to oviposition
sites. If the probability that males will attempt to interfere with oviposition is
high, then even if non-territorial oviposition sites are available, females are
unlikely to be able to use them. Females should copulate with territorial males to
obtain their postcopulatory guarding services. Copulation typically occurs at the
oviposition site and is very brief. Oviposition is also very brief and guarding is by
262
K. F. CONRAD AND G. PRITCHARD
the non-contact method. This permits almost constant male vigilance and allows
males to take advantage of the high encounter rates with females that occur in
this situation. The resource-control mating system is exemplified by Libellulu
(Plathemis) bdiu (Drury) (Campanella & Wolf, 1974; Jacobs, 1955; Koenig &
Albano, 1985, 1987).
Resource-limitation
In the resource-limitation system, males are not able to control the access of
females to all oviposition sites. Some oviposition sites may be too small to attract
enough females to make them profitable for males to defend, or they might have
characteristics that make them good oviposition sites, but not good territorial
sites (e.g. lack of a territorial perching site, Waltz & Wolf, 1984). The duration
of oviposition (Poethke & Kaiser, 1985, 1987) and the type of oviposition
behaviour (Waage, 1984b; Rowe, 1988) may also limit male control of
oviposition sites. In such cases, non-virginal females are able to reproduce
without further male assistance, since they are able to use uncontrolled sites for
oviposition.
In a resource-limitation system, females do not need to mate with a territorial
male to reproduce successfully. Predicting the relative intensities of intersexual
and intrasexual selection is most difficult in this type of mating system. There
should be strong sexual selection on male characters that allow them to obtain
and hold territories. However, since females may also mate with non-territorial
males, female choice may also become important, and there may be strong
sexual selection for characters of morphological or behavioural display.
Furthermore, since signals may serve both inter- and intra-sexual functions,
assigning traits as the products of inter- or intra-sexual selection becomes even
more difficult (Thornhill, 1979; Thornhill & Alcock, 1983). The relative effects
of inter- and intra-sexual selection in such a system may depend on the density of
males, the proportion of oviposition sites that may serve as territories, and the
proportion of females that oviposit without mating with a territorial male.
The resource-limitation strategy is typical of many of the Calopterygid
damselflies (Conrad & Herman, 1987; Johnson, 1962; Miyakawa, 1982;
Pajunen, 1966; Waage, 1973, 1984b). Oviposition sites are clumped but clumps
are fairly regularly dispersed, permitting males to localize around them and
defend them against competitors. The territories thus formed provide a place to
encounter females as they come to the water. However, the price of territoriality
is constant vigilance (Alcock, 1979) and copulations are relatively brief (Corbet’s
(1962) “medium” copulatory duration). This may either permit, or be a result
of, territoriality since it allows males to return quickly to territorial defence.
Postcopulatory guarding is of the non-contact type. This also permits males to
return quickly to territorial defence (Alcock, 1979) and to acquire additional
mates (Waage, 1979b). A female may choose her mate on the basis of his genetic
quality or the quality of the territory he holds, or any combination of the two
(Borgia, 1979). A non-territorial mating strategy may also be expected to arise
because females can obtain access to some non-territorial oviposition sites. A
female may accept a high quality non-territorial male as a mate and attempt to
oviposit at an unguarded oviposition site or even attempt to sneak onto a
guarded site (e.g. Waage, 1979a).
CLASSIFICATION OF ODONATE MATING SYSTEMS
263
Male quality and territory quality in resource-based system
We have presented resource-based systems as a continuum between resource
limitation and resource control. In systems with parental care, the distinction
between these types is not as essential. Emlen & Oring (1977) discuss male
acquisition of multiple mates in terms of their resource holding potential. Males
must hold resources for the duration of the breeding season in order to be able to
reproduce successfully. Females require resources of high enough quality to raise
their offspring and males of high enough quality to defend the resource, and
males often provide care for the duration of the breeding season. Male genetic
quality may therefore be directly related to resource holding potential and may
be indicated by the quality of territory the male holds. Furthermore, indicators
of male genetic quality may directly demonstrate the male’s ability to provide
parental care. When one resource is needed to produce offspring during one or a
limited number of breeding attempts during one breeding season, benefits of
male quality and territory quality to females are not easily separated. The
correlation between male genetic quality and the quality of his resource is likely
to be high and mate choice is likely to be based on the ‘package’ of male and
resource.
In mating systems where there is no parental care, as in odonates, one resource
(oviposition site) may be used by many males and many females (and many
combinations thereof). The determinant of male reproductive success becomes
the time the male controls the oviposition site. The determinants of female
reproductive success are two separate things: the quality of the oviposition site
and the genetic quality of the male at that oviposition site. Genetic quality of a
male is likely to be correlated with how long he can control a resource, but a
female odonate approaching an oviposition site cannot measure the male’s
residency. Furthermore, males encountered at places other than the oviposition
site may be of high genetic quality. Such males may be either too young or too
old to hold a territory (e.g. Forsyth & Montgomerie, 1987), or unable to hold a
territory for some other environmentally determined reason, but this does not
influence or indicate their genetic quality.
When males control the access to all oviposition sites, females should select the
best oviposition site and mate with the male controlling it. When males are
unable to control access to all oviposition sites, females may oviposit without
mating with a male controlling an oviposition site. They may therefore choose
the quality of their mate and their oviposition site separately. The relative
influence of inter- and intra-sexual selection should be related to the relative
importance of genetic factors and oviposition site quality in determining female
reproductive success. The way in which inter- and intra-sexual selection operate
in resource-limitation and resource-control systems therefore differs, and this
differs still from the accepted pattern in resource-based systems with parental
care.
COMPARISONS BETWEEN MATING SYSTEMS
The relationships between the different types of mating systems are illustrated
in Fig. 1 and the characteristics of each are summarized in Table 2.
It is commonly assumed that the intensity of sexual selection on males is
related to the operational sex ratio, especially in the absence of parental care
K. F. CONRAD AND G. PRITCHARD
264
Relationship between the different types
of mating systems
No
Encounter-Llmlted
matinQ
Males able to
control resources?
Free Female
Males able to
controlall access
to resources?
FemaleControl
No
Choice
ResourceControl
Figure I . Flow diagram of the relationship between the different types of mating systems.
(Sutherland, 1985; Emlen & Oring, 1977). From this, it follows that the intensity
of sexual selection on males is least in monogamous species (low operational sex
ratio) and highest in polygynous systems (high operational sex ratio).
The eggs ofodonates mature in successive batches (Corbet, 1962) and in many
species females come to the water only when they have a mature batch of eggs
and are ready to mate. The rapidity of maturation of eggs, the amount of time
the female takes to oviposit a complete clutch and the number of times a female
mates while carrying a batch of mature eggs all influence the availability of
fertilizable ova to males. The operational sex ratio is then a product of both the
number of females and the frequency with which they mate per batch of mature
eggs they carry. Where there is a high degree of sperm precedence, as in many
odonates (Waage, 1986a), all gravid females are potential mates for males,
including those that are in the process of breeding (ovipositing) with another
male. In odonates, therefore, the true operational sex ratio is difficult to measure
because a male’s reproductive success is determined by the number of eggs his
mate lays before mating with another male.
Usually it is predicted that the intensity of sexual selection on males in
territorial species will be higher than in non-territorial species, Where there is
Distribution
of ovipositon
sites
No
Partially Yes
Rare
Frequent
Yes
Very
frequent
No
No
Rare
Yes
No
No
No
No
No
No
Males
Males
Males
able to
defend
perform
limit
mating
courtship
female territories? displays?
access to
resources?
Very rare
Frequency
of malefemale
encounters
*Durations of copulation are based on Corbet (1962).
Resource- Clumped
control
and all
clumps
SENe as
territories
Encounter- Widely
limited
distributed
in a single
habitat
Free female Widely
choice
distributed,
possibly
over many
habitats or
sites
FemaleNumerous,
control
widely
dispersed
within a
single
habitat
Resource- Clumped,
limitation but all sites
not
territories
Mating
system
Yes
Nearby
No
No
No
Unguarded? None?
Usually
guarded
but unguarded
does occur
> 5 min
> 5 min
Based on
Limited
NonYes,
Based on
contact especially territory
for males quality
only
Morphological
or
behavioural
displays,
characters
enabling
territorial
defence
Intrasexual Characters
enabling
territorial
defence
< 1.5 min Guarded
Intrasexual Characters
favouring
matesearching
efficiency
Intersexual Morphological
or
behavioural
displays
Intrasexual? Longevity
NonYes,for
Based on Either
contact both
male
males and genetic or
females
territory
quality or
both
male
genetic
quality
(‘good
genes’)
Rare for
Very
both sexes limited
Unknown
Unlikely
Female
Intra- or
Characters
mate
inter-sexual acted on by
choice? selection presexual
dominant?
selection
1.5-5.0 minBoth, often
at several
sites per
day
Contact
Unguarded? None
> 5 min
Copulation Duration of Guarded
Multiple
Type of
at
copulation*
or
postmatings per
oviposition
unguarded copulatory
day for
site?
oviposition? guarding
either sex
TABLE
2. Summary of the characteristics of each of the five types of d o n a t e mating system
266
K. F. CONRAD AND G . PRITCHARD
opportunity for males to mate with a greater number of females there is usually
greater variance in reproductive success among males and therefore greater
potential for sexual selection among these species. In species in which males are
territorial, males may have several females that they have mated with
ovipositing in their territories at one time, effectively making them mated to
several females at once. Variation in male reproductive success is expected to be
highest in these species. I n species where males contact-guard their mates for
part of the oviposition bout and non-contact guard them for the rest of the bout
(e.g. Fincke, 1982), there is potentially a greater number of matings per batch of
eggs and per day for both sexes compared with those that contact-guard
throughout. Variation in male reproductive success is expected to be slightly
higher in these species than in species with full-time post-copulatory contact
guarding. Non-territorial male odonates usually practise some form of contact
guarding, meaning they can only actually be mated to one female at a time, and
usually to only a limited number of females a day (e.g. Bick & Bick, 1965).
Variation in male reproductive success is expected to be lowest in these species.
However, differences in lifespan, time between maturation of successive
batches of eggs, number of oviposition bouts per lifetime and frequency of
copulations between bouts of oviposition may result in similar variation in
mating success of males among very different mating systems. Ims (1988) has
modelled the effect of spatial distribution of females, temporal distribution of
female receptive periods and sex ratio on variance in male mating success. His
model is based mainly on mammalian mating systems, but he acknowledges its
applicability to insects. Ims (1988) concluded from his model that the temporal
availability of females most strongly affected the variance in reproductive success
in males; variance increased with increased female breeding asynchrony. H e
(Ims, 1988) also showed that female dispersion and sex ratio strongly affected
this pattern and these factors had a greater effect when asynchrony was greater.
As the ratio of females increased or as receptive females became more clumped,
variation in male reproductive success decreased and this effect was increased by
female breeding asynchrony. Predictions of differences in the intensity of
selection on males between mating systems must be made in terms of the
potential variation in the number of eggs fertilized per male, rather than the
potential variation in number of mates a male may have.
CONCLUSIONS
Amongst most odonate mating systems, much of the sexual selection on males
results from male-male competition for access to mates. Sexual selection via
female choice is relatively less important or operates indirectly through females’
choices of times or places to mate. Female choice is more likely to have direct
influence in resource-limitation systems, where females may select mates and
oviposition sites independent of each other, than in resource-control systems,
where females have little opportunity to choose an oviposition site without
mating with the male that defends it. We place resource-control and resourcelimitation at opposite ends of a resource-defence continuum, and postulate that
female choice will have greater influence in a mating system that is more like a
resource-limitation system and less influence in a mating system that is more like
resource-control. Sexual selection is likely to be weak in species that resort to
CLASSIFICATION O F ODONATE MATING SYSTEMS
267
encounter-limited mating. Longevity is likely to contribute strongly to variation
in reproductive success in such species, relative to other male and female
characteristics. Females have limited opportunity to exercise choice among males
in the female-control mating system. Sexual selection on males is most likely to
operate on characters which contribute to their efficiency in searching for and
capturing mates. Predictions about the intensity of sexual selection between
different types of odonate mating systems should be made on the basis of the
variation in the number of potential fertilizations per male or even per ejaculate,
rather than the number of fertilizable females per male. Sperm competition, the
pattern of maturation of eggs and the frequency with which females mate while
depositing a single batch of eggs make the operational sex ratio a poor predictor
of eggs available for fertilization. Because of this, very different mating systems
could result in similar patterns of variation in male reproductive success.
ACKNOWLEDGEMENTS
We thank Drs J. K. Waage and I. Jamieson for comments which greatly
improved the manuscript. H. C. Proctor, Dr R. E. Owen and the members of
the Division of Ecology Seminars of the University of Calgary, the Queen’s
University Biological Station Seminars and the R. J. Robertson laboratory
discussion group all contributed to the development of the paper. Research was
funded by a Natural Sciences and Engineering Research Council graduate
fellowship to KFC and operating grant to GP. Logistic support to KFC during
the writing of the manuscript was provided by Dr R. J. Robertson.
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